Fix transitive dependencies overriding default Kotlin version (KT-28820)

In Gradle 5.0, the 'prefer' function semantics has changed, and now a
transitive dependency version wins over the 'preferred' one. Instead,
'require' has been introduced in 5.0 with the old semantics.

Our users expect that the default Kotlin dependency version will be
at least as new as the plugin version, so we now need to use 'require'
with Gradle 5.0+

Issue KT-28820 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/KotlinGradlePluginIT.kt

@@ -862,6 +862,20 @@ class KotlinGradleIT : BaseGradleIT() {
         }
     }
 
+    @Test
+    fun testDefaultKotlinVersionIsNotAffectedByTransitiveDependencies() =
+        with(Project("simpleProject", GradleVersionRequired.AtLeast("4.4"))) {
+            setupWorkingDir()
+            // Add a dependency with an explicit lower Kotlin version that has a kotlin-stdlib transitive dependency:
+            gradleBuildScript().appendText("\ndependencies { compile 'org.jetbrains.kotlin:kotlin-reflect:1.2.71' }")
+            testResolveAllConfigurations {
+                assertSuccessful()
+                assertContains(">> :compile --> kotlin-reflect-1.2.71.jar")
+                // Check that the default newer Kotlin version still wins for 'kotlin-stdlib':
+                assertContains(">> :compile --> kotlin-stdlib-${defaultBuildOptions().kotlinVersion}.jar")
+            }
+        }
+
     @Test
     fun testNoTaskConfigurationForcing() {
         val gradleVersionRequirement = GradleVersionRequired.AtLeast("4.9")

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/VariantAwareDependenciesIT.kt

@@ -73,7 +73,10 @@ class VariantAwareDependenciesIT : BaseGradleIT() {
         val outerProject = Project("sample-lib", gradleVersion, "new-mpp-lib-and-app")
         val innerProject = Project("simpleProject").apply {
             setupWorkingDir()
-            gradleBuildScript().modify { it.replace("apply plugin: \"kotlin\"", "") }
+            gradleBuildScript().modify {
+                it.replace("apply plugin: \"kotlin\"", "")
+                    .replace("\"org.jetbrains.kotlin:kotlin-stdlib\"", "\"org.jetbrains.kotlin:kotlin-stdlib:\$kotlin_version\"")
+            }
         }
 
         with(outerProject) {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/simpleProject/build.gradle

@@ -24,7 +24,7 @@ dependencies {
     compile 'com.google.guava:guava:12.0'
     deployCompile 'com.google.guava:guava:12.0'
     testCompile 'org.testng:testng:6.8'
-    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
+    compile "org.jetbrains.kotlin:kotlin-stdlib"
 }
 
 test {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -6,6 +6,7 @@ import com.android.builder.model.SourceProvider
 import groovy.lang.Closure
 import org.gradle.api.*
 import org.gradle.api.artifacts.ExternalDependency
+import org.gradle.api.artifacts.MutableVersionConstraint
 import org.gradle.api.attributes.Usage
 import org.gradle.api.file.FileCollection
 import org.gradle.api.file.SourceDirectorySet
@@ -502,11 +503,21 @@ internal abstract class AbstractKotlinPlugin(
 internal fun configureDefaultVersionsResolutionStrategy(project: Project, kotlinPluginVersion: String) {
     project.configurations.all { configuration ->
         if (isGradleVersionAtLeast(4, 4)) {
+            fun MutableVersionConstraint.chooseVersion(version: String) {
+                if (isGradleVersionAtLeast(5, 0)) {
+                    // In Gradle 5.0, the semantics of 'prefer' has changed to be much less imperative, and now it's 'require' that we need:
+                    val requireMethod = javaClass.getMethod("require", String::class.java)
+                    requireMethod(this, version)
+                } else {
+                    prefer(version)
+                }
+            }
+
             // Use the API introduced in Gradle 4.4 to modify the dependencies directly before they are resolved:
             configuration.withDependencies { dependencySet ->
                 dependencySet.filterIsInstance<ExternalDependency>()
                     .filter { it.group == "org.jetbrains.kotlin" && it.version.isNullOrEmpty() }
-                    .forEach { it.version { constraint -> constraint.prefer(kotlinPluginVersion) } }
+                    .forEach { it.version { constraint -> constraint.chooseVersion(kotlinPluginVersion) } }
             }
         } else {
             configuration.resolutionStrategy.eachDependency { details ->